Pneumatic actuation systems, which generally comprise a piston controlled by one or more pressure-control valves, are used in certain industrial applications. Recently, Aurora Flight Sciences Corporation (“Aurora”) of Manassas, Va. incorporated pneumatic actuation systems into aircraft control systems. For example, Aurora recently developed a right-seat pilot assistant capable of operating an aircraft during takeoff, cruise, and landing using pneumatic actuation to manipulate primary flight controls. Despite mechanical simplicity, the dynamics of pneumatic actuation systems are intrinsically non-linear because of the switching nature of pressure-control valves and non-linear nature of the airflow that governs pressure change in the cylinder itself.
Conventional pneumatic actuation system controller design methods can be categorized into two general categories. The first design method employs a standard proportional integral derivative (PID) control architecture in which the gains are chosen experimentally. That is, there is little use of the extensive control theory available for robust design of the controller. The primary feedback signal on which the control compensator operates is the position error, which is defined as the difference between the commanded and actual position of the piston. A second design method employs complex non-linear methods, such as sliding-mode control. In these implementations, some signals are used for system feedback, but the gain-selection or control-design process is still based largely on heuristics and/or experimentation, which can be expensive, inaccurate, and overly complicated.
It is often advantageous to employ redundancy techniques such that a fault of one actuator will have only a nominal effect on the overall system. Such redundancy typically involves the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of a backup or fail-safe, or to improve actual system performance. In aerospace applications, for example, safety-critical systems (e.g., fly-by-wire, hydraulic systems in aircraft, flight control systems, etc.) may be triplicated. In a triplex-redundant (aka, triply redundant) system, the system has three subcomponents, all three of which must fail before the overall system fails. Since each subcomponent rarely fails, and because the subcomponents are expected to fail independently, the probability of all three subcomponents failing is calculated to be extraordinarily small.
In view of the foregoing, a need exists for a pneumatic actuation controller architecture that offers additional feedback, without requiring complex heuristics and/or experimentation. More specifically, a need exists for a triplex-redundant architecture for pneumatic actuation in safety-critical aerospace applications.